Fix Ruby 2.7 deprecation warn on Mounter#remove?

On Ruby 2.7, with deprecation warnings enabled, the Mounter#remove? code
which handles string arguments to remove that we want to be treated as
false-y ("0" and "false") was throwing a deprecation warning:

"warning: deprecated Object#=~ is called on TrueClass; it always returns
nil"

This change fixes that warning by checking if the value is TrueClass
before running the regular expression.

Note that this does not change any behaviour since the presence check
was already handling the case that a false Boolean was passed, and the
case that a true Boolean was passed was already correctly not matching
the regular expression albeit with the deprecation warning.

This fix is the same as the one on the master branch commit
9a37fc9 by mshibuya.

lib/carrierwave/mounter.rb

@@ -110,7 +110,7 @@ def blank?
     end
 
     def remove?
-      remove.present? && remove !~ /\A0|false$\z/
+      remove.present? && (remove == true || remove !~ /\A0|false$\z/)
     end
 
     def remove!